The present invention pertains to a sound reproducing system. More particularly, it relates to a ducted-port bass-reflex loudspeaker arrangement.
For a long number of years, much effort has been devoted in attempts to improve the fidelity of sound systems employed particularly for the reproduction of recordings, broadcasts and live entertainment. In one area of this effort, a considerable amount of work has been expended, both empirically and theoretically, with an aim toward better low-frequency or bass response while yet avoiding the use of speaker systems which become so large as to be unwieldy and, in some cases, impractical. While important improvements have been made in this regard, it is still insisted in many quarters that truly faithful bass response cannot be obtained without utilizing a comparatively large diameter loudspeaker combined with a very much larger enclosure. Somewhat typifying the discussions in the literature is an article entitled "Wisdom and Witchcraft of Old Wives' Tales about Woofer Baffles" by J. R. Ashley and T. A. Saponas that appeared in the October 1970 issue of the Journal of the Audio Engineering Society, Volume 18, No. 5, pp. 524-529. Of similar import are an article entitled "Improvement in `Air Suspension` Speaker Enclosures with Tube Venting" by P. B. Williams and J. F. Novak, commercially published by the Jensen Manufacturing Company, a division of the Muter Company of Chicago, Ill., and an article entitled "Performance of Enclosures for Low-Resonance, High-Compliance Loudspeakers" by J. F. Novak which appeared in I.R.E. Transactions on Audio for January-February, 1959, Volume AU-7, No. 1, at Pages 5-13. Also of interest is an article under the title "Loudspeakers in Vented Boxes" by A. N. Thiele that was published in the Journal of the Audio Engineering Society, Volume 19, for May-June, 1971.
One approach often mentioned in the foregoing and other publications involves the use a ducted-port bass-reflex enclosure for the loudspeaker. In that approach, the loudspeaker is mounted in an opening in a cabinet which is air-tight except for a second opening from which a tube or duct projects inwardly into the cabinet. It has been shown that use of the bass reflex principle enables the attainment of a three to four decibel increase in efficiency over the more conventional air-suspension closed-boxed type of enclosure. One possible way of designing this type of enclosure is described in an article entitled "Designing a Ducted-Port Bass-Reflex Enclosure" by James F. Novak, published in 1965 commercially by the aforementioned Jensen Manufacturing Division of the Muter Company.
At least usually, both the loudspeaker and the enclosure in prior ducted-port bass reflex systems have been tuned to approximately 30 hertz. The enclosure is quite small, perhaps between one and two cubic feet, and the loudspeaker has a comparatively light moving mass. The result is the combined production of a sound pressure level which increases generally from below 30 hertz to about 90 hertz beyond which the sound pressure level is comparatively constant throughout the low-frequency range. The purpose of the ducted port in these prior systems is to provide acoustical loading of the loudspeaker cone at low frequencies in an effort to reduce distortion. However, the output sound pressure level of such systems at the tuned frequency is generally between ten and eighteen decibels down from the sound pressure level over the higher frequencies in the bass region. That is, the loudspeaker itself is radiating efficiently at a certain pressure level only down to about 90 hertz, while the ducted port, although contributing to radiation of the sound at lower frequencies, is producing that lower-frequency sound only at a substantially reduced pressure level.